According to technical standard specifications of a mobile communication system specified by 3rd Generation Partnership Project (3GPP), a local ringing tone to be outputted based on data stored in a call-originating terminal (mobile station) and a NW-provided ringing tone to be transmitted to the call-originating terminal from a network are specified as ringing tones to be outputted to a user during alerting of a call-terminating terminal. Also, a CAT (Customized Alerting Tone) using the user's favorite music or the like as the ringing tone is specified as one of the NW-provided ringing tones (e.g., Non-patent document 1).
The local ringing tone is outputted from a speaker or the like, based on the data stored in the call-originating terminal (mobile station). On the other hand, as to the NW-provided ringing tone, a ringing tone (RBT/CAT) transmitted as Early Media (specified in IETF RFC 3960 or the like) to the call-originating terminal from the network is outputted from the speaker. To be more specific, the NW-provided ringing tone is outputted at a time when “180 Ringing” according to Session Initiation Protocol (SIP) is transmitted to the call-originating terminal from the call-terminating terminal.
Moreover, in the 3GPP technical standard specifications, SRVCC (single Radio Voice Call Continuity) is specified, which allows a mobile station executing voice communication via Long Term Evolution (LTE) with no circuit-switched domain to make a transition to a 3G (W-CDMA) circuit-switched domain while continuing the voice communication (e.g., Non-patent document 2). As a type of the SRVCC, aSRVCC (SRVCC in alerting phase) is also specified, which enables a transition from LTE to 3G during alerting of the call-terminating terminal. The aSRVCC realizes the transition from LTE to 3G while maintaining the alerting even during the alerting of the call-terminating terminal.